The ability to store electrical energy is widely recognized as a key limiting factor in the widespread use of renewable energies such as wind and solar, which are intrinsically intermittent in their supply capabilities. Electricity storage is also recognized as a necessity in an electrical grid to store excess (base) supply during hours of low demand for release during hours of high (peak) demand.
There are many techniques available for storing excess electrical energy for later release. A comprehensive account of the various available electrical energy storage techniques is provided in U.S. Pat. No. 7,832,207 (the '207 patent), the disclosure of which is hereby incorporated herein by reference in its entirety.
Compressed air energy storage (CAES) is an effective means of storing electrical energy and has a long history of development. The main challenge in accomplishing a high level of energy storage and recovery efficiency is to either maintain the temperature relatively constant during both compression (energy storage) and expansion (energy release) in an isothermal process, or store the heat generated during compression to reheat the gas during expansion in an adiabatic process. An isothermal process has an intrinsically much higher energy storage and recovery potential than an adiabatic process, and is therefore more desirable. However, an isothermal process requires provisions for adequate exchange of large quantities of heat between the gas and its environment, which requires heat exchange systems that add complexity and cost. Also, an isothermal process cannot directly use the compressed air to drive an electrical generator such as in a turbine, as the rapid expansion of the gas is inherently adiabatic. Current approach is to use a liquid as the working fluid in a hydraulic-pneumatic accumulator and intensifier arrangement and thereby avoid rapid adiabatic gas expansion for power generation. The hydraulic-pneumatic accumulator and intensifier arrangement is intricate and requires special hardware and software that add complexity and cost to the overall CAES system.
Current hydraulic-pneumatic CAES systems are either closed-air in which the gas is sealed within a cylinder and is never expanded to or compressed from atmospheric pressure, or open-air in which the air is pressurized from atmospheric pressure and expanded back to atmospheric pressure by venting to open air. Closed-air systems have the desired feature of being sealed and thus protected against moisture and dust in the compressed air since they are charged only once in a controlled environment with a certain mass of clean dry air. However, current closed-air systems are not economical because practical limits on the size and maximum pressure of conventional cylinders limit their energy storage capability. To make existing closed-air systems capable of significant energy storage, many large cylinders would be needed requiring extensive land. Open air hydraulic-pneumatic accumulator and intensifier arrangements utilize specialized hydraulic machines and control software with complex valve switching sequences. These systems are recent and currently under development such that they do not have an operational track record.
Using a direct heat-exchange subsystem with cylinders, such as an external jacket, to simplify existing systems and reduce cost is not currently effective because existing air compression technologies use successive rapid compressions of small quantities of gas that generate heat and therefore require an equally rapid rate of heat transfer to maintain isothermal conditions, which far exceeds the available rate of heat exchange across the cylinder perimeter area.
Therefore, there is a need for a simple and practical system for compressing and expanding gas isothermally, without the need for separate heat exchange subsystem and hydraulic-pneumatic accumulator and intensifier arrangement. Such a system would reduce complexity and cost electrical energy storage making its widespread use feasible.